Things to understand about unRAID and XEN.

There seems to be a little misconception about the target audience for unRAID and XEN, so I think I’ll take a couple minutes and clarify.

unRAID is primarily aimed at the entry to mid level as far as difficulty is concerned. Also you need some technical know how and the capability to trouble shoot. This is not a Western Digital DUO or equivalent, it is not plug and play. This software is aimed at the DIY crowd, having said this you don’t need a lot of technical ability as there is a lot of documentation out there for unRAID and the community is one of the best I have ever had the privilege to be a part of. Also to note that unRAID 6 is in beta right now and as of yet it does not have any plugins written for it, much like version 4 and 5 the finished product will have these. If you do not want to use XEN, in the final product you won’t have to. Right now however since it is in beta if you want application on version 6 you need to run them in a VM such as Ubuntu and Arch Linux. unRAID can be run on very inexpensive hardware, and it can be scaled as required by the user. Some users run unRAID on Atom processors with 4GB of RAM. If you do this don’t expect stellar performance.

This is not FreeNAS. I found FreeNAS to be flaky at best, and the hardware requirements for that OS are very particular. ZFS has some nice features and it protects your data really well, however it requires ECC RAM and quite a bit of it too. Cost of FreeNAS can be greater than unRAID. I say “Can be..”, because you can throw any hardware at unRAID, be it low end or high end it really doesn’t matter. So in the end unRAID scales better and one can start out with cheap hardware, then slowly upgrade. Also with unRAID and XEN now you can have features such as the ones offered in ZFS to protect your critical data, it is just a matter of you willing to put the work in.

Each disk in the unRAID array is independent, two disks crashing will not bring down your entire array only the disks that crashed. The array is designed only for parity. Parity is used by unRAID to protect against data loss. If a drive in the array fails, the data on the other drives can be combined with the parity data to reconstruct the missing data.In general, a parity process is designed to detect a single bit change across a given set of bits, by setting the value of an added bit such that a summation across that set of bits is forced to a known value. The added bit is known as a parity bit.

Digital data is stored as a 1 or a 0. So, for example:

  • If you have 4 drives with bit values 1,1,1,1 the parity will be 0 (1+1+1+1+0=even).
  • If the bit values are 1,0,0,0 the parity will be 1 (1+0+0+0+1=even).

In unRAID, the parity bits are stored on a parity drive independent of the data drives. This parity bit works across the set of bits in the same relative bit position on each drive. So the 57th bit of the parity drive is the parity bit for the 57th bit of all of the data drives. A parity check of the 57th bit position therefore adds up all of the 57th bits of every drive including the parity drive, and checking to see total is an EVEN number. If the value returned is not EVEN, then the parity bit is toggled so that the parity check WILL return an EVEN number. unRAID uses ‘even parity’, which simply means that the summation process (using a mathematical operation called ‘exclusive OR’ or ‘XOR’) across that set of bits must return a value that is an EVEN number.

XEN hypervisor is a lot more difficult to use than any other, IMO. However the benefits of the hypervisor outweigh the effort necessary to get XEN going. If you do not need a hypervisor go with unRAID 5 or 4, the license is transferable to version 6. Why XEN? XEN is capable of running a Windows VM that you can game on, provided you have the necessary hardware and are running a HVM. So it is very robust and powerful.


4 thoughts on “Things to understand about unRAID and XEN.

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  1. Hi, I’m researching various peoples’ experiences with ZFS. Would you please share what motherboard, CPU, RAM (how many GB, ECC or non-ECC?), HBA, NIC, and hard drives you were using with FreeNAS? Were you using the system as a dedicated NAS, or was it doubling as a desktop or hypervisor? How many GB of RAM were provisioned to the VMs, and how many were left for the host?

    Best regards,

    1. Nick,

      I abandoned FreeNAS rather quickly, so I can’t comment on nor do I have much of a personal experience. I was researching weather or not FreeNAS would be a viable solution for me, but it was not. After getting the FreeNAS setup I could not start the BtSync service, which was a requirement for my environment. At this point I moved back to unRAID where I was able to run BtSync in a Arch Linux VM without issues.

      Originally I wanted to run this on a core i3 with 4 GB of RAM as a stand alone NAS device, with OwnCloud and BtSync.


      1. Thanks for the info! I’m surprised you didn’t try out FreeBSD given that you like Arch. 😉 Unraid is pretty slick stuff, but but I can’t risk it glitching and then propagating to backups. ‘depends how important your data is, you know? Last I tried SnapRAID, in an experiment for poor man’s data integrity, it was very labour intensive to recover from errors…given the hours I poured into that, I’m thinking the smartest thing to do is migrate to ZFS+ECC in ZRAID2/RAID6, and then sell my existing hardware, since time is money. The cheapest a server with 12TB+6TB parity, running on Intel hardware (CPU, NIC, HBA) with 16GB ECC RAM can be had is ~1600$ CAD. $160/TB seems like a fair price for near bulletproof storage. I’ll probably go with a FreeBSD, rather than an OpenIndianna derivative…ZoL isn’t ready yet imho, btrfs is definitely not ready, and ZBOSS is a brand-new mystery. Sorry for the delay replying, btw, life’s been insane here–transcontinental move.

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